(1) Field of the Invention
The present invention relates to a method of fabricating semiconductor devices, and more particularly to a method of forming a capacitor element in super-LSIs such as DRAMs (Dynamic Random Access Memories).
(2) Description of the Related Art
With respect to the capacitor of super-LSI memory devices over 256 MB (megabytes) DRAM, researches and investigations have been made for the adoption of high dielectric capacitive insulating films which permit increasing the capacitance per unit area. Among the high dielectric capacitive insulating films that are being studied, the tantalum oxide film formed by a chemical vapor deposition (CVD) process has high specific dielectric constant .epsilon..sub.r of 25 to 30 and an excellent step coverage characteristic. Further, its film formation process is extremely easy compared to that in other high dielectric capacitive insulating films. For these reasons, extensive researches are being carried out in this field of technology.
FIGS. 1A, 1B and 1C show, in sectional views, successive steps of The conventional process of forming a capacitor element using a tantalum film.
First, as seen in FIG. 1A, polysilicon is deposited by the CVD process on a p-type silicon substrate 1, which has an n-type diffusion layer la at its surface region, and on which an element isolation region 2 having an opening reaching the n-type diffusion layer la is formed. Then, phosphorus (P) is thermally diffused, and then a polysilicon inner or lower electrode 3 is formed by a usual lithographic technique. In this stage, a natural oxide film 4 is formed on the surface of the polysilicon electrode 3.
Subsequently, as seen in FIG. 1B, a tantalum oxide film 7 is formed on the polysilicon lower electrode 3 by a low-pressure chemical vapor deposition (LPCVD) process using ethoxytantalum (Ta(OC.sub.2 H.sub.5).sub.5) as a source gas. The wafer is then subjected to a high temperature heat treatment in oxygen atmosphere at 600.degree. to 1000.degree. C. to reduce leakage current, thereby improving the leakage current characteristics in the tantalum oxide film 7. At this time, the natural oxide film 4 becomes a SiO.sub.2 film 4a. Subsequently, as shown in FIG. 1C, an outer or upper electrode 6 is formed. For the upper electrode 6, tungsten (W) is generally used. Through the above steps, the formation of the capacitor is completed.
The above prior art capacitor structure has the following problems. In the prior art capacitor formation process, the capacitor that is formed by forming the tantalum oxide film 7 on the polysilicon as the lower electrode 3, followed by the high temperature heat treatment in oxygen atmosphere to improve the leakage current characteristics, has a capacitance of only up to about 3 nm in thickness (Cs=12 pF/.mu.m.sup.2) in terms of the equivalent thickness converted into SiO.sub.2 film (specific dielectric constant .epsilon..sub.r =3.9). This is so because, due to the high temperature heat treatment performed in oxygen atmosphere to improve the leakage current characteristics of the tantalum oxide film, the natural oxide film 4 present at the interface between the tantalum oxide film 7 and the polysilicon electrode 3 is increased in thickness and becomes the SiO.sub.2 film 4a. Where this capacitive insulating film is used in a capacitor over 256 MB DRAM, a sufficient capacitance cannot be obtained. Another problem is that the capacitor element formed in the prior art has a leakage current characteristic (10.sup.-8 A/cm.sup.2) with a low voltage of about 0.7 V as can be seen in FIG. 6. The capacitor element having such leakage current characteristics cannot be applied to any practical device.